The highest speed and therefore most commonly employed photographic elements are those which contain a radiation-sensitive silver bromide or bromoiodide emulsion layer coated on a support. Although other ingredients can be present, the essential components of the emulsion layer are radiation-sensitive silver bromide microcrystals, optionally containing iodide, commonly referred to as grains, which form the discrete phase of the photographic emulsion, and a vehicle, which forms the continuous phase of the photographic emulsion.
Interest in silver halide photography has recently focused on tabular grain emulsions, particularly thin intermediate and high aspect ratio tabular grain emulsions. It has been shown that these emulsions can produce a variety of photographic advantages, including increased sharpness, improved speed-granularity relationships, increased blue and minus-blue speed separations, more rapid developability, higher silver covering power when fully forehardened, reduced crossover in spectrally sensitized dual coated (also referred to as two sided or Duplitized.RTM.) radiographic formats, and various imaging advantages in dye image transfer film units. Research Disclosure, Vol. 225, January 1983, Item 22534, is considered representative of these teachings, Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England.
It is well known that silver halide emulsion preparation includes the stages of nucleation and growth. In the nucleation stage, new crystals of minute size are created. The growth stage involves addition of new material to existing crystals. These stages are distinct from the process of Ostwald ripening in which large crystals grow at the expense of small ones which are more soluble.
Both the nucleation and growth stages of silver halide emulsion preparation require the use of a peptizer to avoid the coalescence or flocculation of the silver halide grains. The vehicle encompasses both the peptizer and the binder employed in the preparation of a silver halide emulsion and the same material or different materials can be used to perform the functions of peptizer and binder.
While a variety of hydrophilic colloids are known to be useful peptizers, the most commonly employed peptizers are gelatin--e.g., alkali-treated gelatin (cattle bone or hide gelatin) or acid-treated gelatin (pigskin or cattle bone gelatin)--and gelatin derivatives--e.g., acetylated gelatin or phthalated gelatin. Gelatin and gelatin derivative peptizers are hereinafter collectively referred to as "gelatino-peptizers."
Materials useful as peptizers, particularly gelatin and gelatin derivatives, are also commonly employed as binders in preparing an emulsion for coating. However, many materials are useful as vehicle extenders, such as latices and other hydrophobic materials, which are inefficient peptizers.
The use of a gelatino-peptizer in preparing thin tabular grain silver bromide or bromoiodide emulsions is described in Maskasky, U.S. Pat. No. 4,713,320, issued Dec. 15, 1987. While the gelatino-peptizers are very effective in preparing such tabular emulsions, they suffer from certain serious disadvantages. Thus, for example, gelatino-peptizers frequently contain impurities which hinder the ability to consistently prepare reproducible emulsions with consistent properties. Specifically, gelatin, which is a derivative of naturally occurring collagen, is very heterogeneous; containing a wide variety of molecules representing triple and double helices, single strands and fragments, as well as impurities such as nucleic acids, fats and non-gel proteins such as cystine and cysteine. Gelatino-peptizers may also lack sufficient resistance to bacterial decomposition and may not permit the use of as wide a range of dopants or chemical or spectral sensitizers as is desirable. In contrast with gelatino-peptizers, synthetic polymeric peptizers provide peptizer molecules that are uniform and can be optimized for specific desirable properties such as silver halide binding strength, solubility, metal ion complexing strength and ionic charge. A further advantage of synthetic polymeric peptizers is greater ease in transferring silver halide emulsions prepared in water to non-aqueous coating formats. After an emulsion has been prepared with the aid of one or more synthetic polymeric peptizers, gelatin can be added to serve as the binder so that the resulting emulsion can be handled in a conventional manner in a photographic system. Thus, synthetic polymeric peptizers have many advantages in the preparation of silver halide emulsions, including thin tabular grain silver halide emulsions. However, many synthetic polymers are very inefficient peptizers and the photographic art has long sought to develop synthetic polymers that would function in an effective manner as peptizers for silver halide grains.
One of the inefficiencies that has been encountered in the prior art in the preparation of tabular grain silver bromide and bromiodide emulsions is the presence of unwanted grain shapes. In addition to unwanted non-tabular grains, also in evidence are thick tabular grains, which have aspect ratios closely approaching those of non-tabular grains.
In addition to low aspect ratio tabular grains and non-tabular grains, these tabular grain emulsions also contain a significant population of grains which are in the form of rods. Because of their length and limited projected areas, rods are of marginal photographic utility. Beyond this, their presence in emulsions is disadvantageous in conventional procedures for manufacturing photographic elements containing silver halide emulsion layers.
It is also known that the introduction of iodide ions during the precipitation of tabular grain emulsions results in thickening of the tablular grains. Thus, when tabular grain silver bromide and silver bromoiodide emulsions precipitated under similar conditions and having similar mean grain diameters are compared, the tabular grain silver bromide emulsions exhibit higher average aspect ratios.
In light of the above, it is apparent that there is a critical need in the art for more effective synthetic polymeric peptizers in order to take advantage of the many benefits which synthetic polymers exhibit in comparison with gelatino-peptizers. There is particularly a critical need for synthetic polymeric peptizers capable of providing thin tabular grain emulsions of the bromide and bromoiodide type. The ability to replace gelatino-peptizers in at least one of the stages of nucleation and growth would represent a significant advance in the art of manufacturing thin tabular grain bromide and especially bromoiodide emulsions.
It is toward the objective of providing an improved process for preparing a thin tabular grain silver halide emulsion, which utilizes a synthetic polymeric peptizer and does not require the use of a gelatino-peptizer, that the present invention is directed.